1. Field of the Invention
The present invention relates to shaped charge tools for explosively severing tubular goods including, but not limited to, pipe, tube, casing and/or casing liner.
2. Description of Related Art
The capacity to quickly, reliably and cleanly sever a joint of tubing or casing deeply within a wellbore is an essential maintenance and salvage operation in the petroleum drilling and exploration industry. Generally, the industry relies upon mechanical, chemical or pyrotechnic devices for such cutting. Among the available options, shaped charge (SC) explosive cutters are often the simplest, fastest and least expensive tools for cutting pipe in a well. The devices are typically conveyed into a well for detonation on a wireline or length of coiled tubing.
Typical explosive pipe cutting devices comprise a consolidated wheel of explosive material having a V-groove perimeter such as a V-belt drive sheave. The circular side faces of the explosive wheel are intimately formed about an axis of revolution against circular metallic end plates. The external surface of the circular V-groove is clad with a thin metal liner. An aperture along the wheel axis provides a receptacle path for a detonation booster.
This V-grooved wheel of shaped explosive is aligned coaxially within a housing sub and the sub is disposed internally of the pipe cutting subject. Accordingly, the plane that includes the circular perimeter of the V-groove apex is substantially perpendicular to the pipe axis.
When detonated at the axial center, the explosive shock wave advances radially along the apex plane against the V-groove liner to drive the opposing liner surfaces together at an extremely high velocity of about 30,000 ft/sec. This high velocity collision of the V-groove liner material generates a localized impingement pressure within the material of about 2 to 4×106 psi. Under pressure of this magnitude, the liner material is essentially fluidized.
Due to the V-groove geometry of the liner material, the collision reaction includes a lineal dynamic vector component along the apex plane. Under the propellant influence of the high impingement pressure, the fluidized mass of liner material flows lineally and radially along this apex plane at velocities in the order of 15,000 ft/sec. Resultant impingement pressures against the surrounding pipe wall may be as high as 6 to 7×106 psi thereby locally fluidizing the pipe wall material.
Traditional fabrication procedures for shaped charge pipe cutters have included an independent formation of the liner as a truncated cone of metallic foil. The transverse sections of the cone are open. In a forming mold with the liner serving as a bottom wall portion of the mold, the explosive is formed or molded against the concave conical face of the liner. At the open center of the truncated apex of the liner, the explosive is formed against the mold bottom surface and around a cylindrical core.
With the precisely desired explosive material in place, an end plate is aligned over the cylindrical core and pressed against the upper surface of the explosive material at a controlled rate and pressure in the manner of a press platen. When removed from the forming mold, the unified liner-explosive-backing plate comprises half of a shaped charge pipe cutter.
To complete a full cutter unit, two of the shaped charge half sections, separated from the cylindrical core mold, are joined along a common axis at a contiguous juncture plane of exposed explosive at the truncated apex face planes. A detonation booster is inserted along the open axial bore of the unit left by the molding core. This detonation booster traverses the half charge juncture plane to bridge the explosive charges respective to the two half sections between the opposing end plates. The charged cutter is inserted into a cutter housing that is secured to a cutter sub.
A notable characteristic of secondary order explosives of the type used in shaped charge cutters such as RDX and HMX is that the detonation velocity roughly corresponds to the compression density of the charge. A greater charge density generally increases the detonation velocity. Hence, more densely compressed charges, generally, are more energetic, emit greater velocity jets and generate greater cutting pressure. However, another characteristic of densely compressed high explosives is a greater difficulty to detonate. It has been a general rule of practice, therefore, that more densely compressed, energetic charges require larger, more intimately positioned ignition boosters.
Larger boosters, for more densely compressed explosives, introduce other complications to the downhole tubing cutter design. Larger boosters require larger diameter axial apertures in the cutter explosive geometry thereby reducing the available volume within the explosive material envelope for high explosive material.
It must be recognized that for a given nominal pipe size, there is a corresponding inside diameter. A cutter housing, meaning the housing outside diameter, must fit loosely within the inside diameter of the pipe that is to be cut. The outside diameter of the cutter explosive wheel must fit within the housing and the outside diameter of the explosive wheel substantially dictates the depth of the liner V-groove.
As the dimensional restrictions progress radially inward, a final distance absolute arises between the inside diameter wall of the booster aperture and the V-groove apex. The radial depth of this annular plane between the V-groove apex and the aperture wall is characterized as the “induction” distance. If insufficient, the explosive detonation will not decompose the liner material into a lineal cutting jet. There is advantage, therefore, for using the smallest diameter booster (and, hence, aperture diameter) that will reliably detonate the cutter charge.
International standards of transportation safety (UN Recommendations on the Transport of Dangerous Goods, Section 16) require that high order explosives such as HMX and RDX are packaged in a manner to promote deflagration rather than explosion upon uncontrolled heating as in an accidental fire. In general, compliance with this regulation precludes any sealed enclosure or confinement of the cutter explosive. If heated, an unconfined explosive will simply out-gas and burn. If the explosive is confined, however, the gas may develop sufficient pressure to initiate a detonation. Hence, in the interest of safety, there should be a gas venting route in any transport packaging.
To comply with these safety requirements, shape charge cutter equipment is therefore transported to a job site in various degrees of disassembly.
Unfortunately, the environmental circumstances of a drilling rig floor, which is where final cutter assembly must occur, are often hostile and usually not conducive to the attentive care required for final assembly of a high explosive tool. Hence, there are strong incentives to transport a cutter unit to the job site in the greatest degree of assembly that safety, prudence and regulation allow.
A representative cutter assembly usually requires the shaped charge explosive to be positioned within an environmental housing which is atmospherically open and unsealed for transport. When finally assembled for downhole placement and detonation, an explosive booster charge is positioned in the axial aperture through the explosive cones. The cutter housing is secured to a top sub which seals the housing enclosure. The housing and top sub are secured to a firing head having an electrically initiated detonator and a capacitive discharge circuit. Upon final assembly for downhole placement and detonation, the housing, top sub and firing head are secured together as a firing unit. When assembled, the detonator is physically positioned in ignition proximity to the booster and the combination of housing, top sub and firing head is totally sealed from the environment outside the housing wall. In process sequence, surface signals prompt a capacitive discharge circuit to electrically discharge into the detonator. The detonator discharge initiates the booster within the axial aperture proximate of the explosive cone interface. The booster ignition detonates the explosive cutter cones.
Each of these firing unit assembly joints is hydraulically sealed by an O-ring. As normally fabricated, however, there is an open channel space along the axis of the assembled unit. Consequently, the opportunity exists at each of the assembly joints for external pipe bore fluid to enter the open channel space and corrupt the shaped charge explosive in the event of O-ring seal failure. This opportunity is exacerbated by rough or poorly machined seal surfaces.
The mechanics of O-ring sealing includes a pressure differential induced distortion of the polymer material from which the O-ring is made. Under a high pressure differential, these principles are extremely reliable. Under a low pressure differential, a fluid tight seal is much more problematic if the seal surfaces are roughly machined or corrupted by deposits. If the pressure differential upon the O-ring is insufficient to force-flow the O-ring polymer material into intimate sealing contact withal of the sealing surface, fluid will by-pass the seal and enter the forbidden zone. For explosive tools such as shaped charge cutters and perforators, such low pressure leakage may be disabling.
Curiously, in a deep well environment, a tool with a low pressure leak may ultimately acquire a complete seal as the tool descends into realms of greater pressure.
To further simplify the job site assembly task, it would be helpful, therefore, to eliminate the need for an explosive booster thereby initiating the cutter explosion only by the firing head detonator. It would also be helpful to provide an internal fluid seal means along the internal channel of the assembly firing unit.
Over years of experience, use and experimentation, the explosion dynamics of shaped charge cutters has evolved dramatically. Some prior notions of critical relationships have been revealed as not so critical. Other notions of insignificance have been discovered to be of great importance. The summation of numerous small departures from the prior art traditions has produced significant performance improvements or significant reductions in fabrication expense.